The manufacture of a cellulosic product such as a sheet of paper from a pulp slurry includes forming portions, pressing portions, and drying portions. Forming the cellulosic product (for example, the sheet of paper) can involve the removal of water by forming section drainage, pressing, and drying. There has been much work conducted in exploring mechanical processes of enhancing removal of water prior to the drying in order to reduce the amount of energy needed for the drying process. As such, there is a need for improvements in the forming section drainage and pressing section drainage that do not require substantial capital investment.
As paper machines age and speed requirements increase, the machines tend to become limited by the drying portions and/or pressing portions. This limits the rate at which water can be removed. Speed above a predetermined rate produces a sheet with higher than desirable moisture levels.
Dewatering advancements in the forming portions and pressing portions have generally been mechanical. A higher dryness coming from the forming portions into the pressing portions may lead to a higher dryness exiting the pressing portions and leads to a lower water load entering the drying portions, thus allowing for a savings in energy or an increase in production.
Pressing portions can be the last chance to increase the dryness of the sheet before entering the drying portions. In pressure controlled pressing portions, the resistance to flow between the fibers of the sheet is insignificant. The dryness of the sheet is dictated by the flow of water exiting the fiber wall. Water in the controlled pressing portions involves a flow phenomenon with the press impulse being the major driving force. Water is removed proportional to the water load of the sheet at a maximum operational pressure. A greater dryness can be achieved by increasing the pressure applied to the sheet, however, above the maximum operational pressure, the structural integrity of the sheet is overcome and the sheet is crushed, creating a lower quality product. Pressure controlled pressing portions apply to single felted presses with basis weights up to 100 g/m2 and to double felted presses with basis weights up to 150 g/m2. The pressure controlled pressing portions can be extended to heavier sheets at higher speeds with modern shoe presses.
Conversely, flow controlled pressing portions are defined by conditions where the rate of water removal is constant at a given set of pressing parameters. This is a sign of poor operational pressing conditions. In the flow controlled pressing portions, water removal follows Darcy's law, as dryness is a function of the press impulse with no independent effect of pressure or time. This condition arises when the water which is being pressed from the sheet is removed at a slower rate than it is created, defeating the purpose of applying a greater pressure. To overcome the limitation, the rate at which water is carried away from the pressing zone is increased in order to achieve a greater dryness out of the press and to revert to the pressure controlled regime.
In Stratton R. A., Use of polymers in wet pressing, Tappi Proceedings Papermakers Conference, pp. 179-185 (1982), hereinafter “Stratton,” which is incorporated by reference in its entirety, various cationic wet end polymers were utilized to demonstrate that increases of up to about 1% to 2% solids out of a press section are possible by addition of the polymers to the wet end. Stratton focused solely on the use of the polymers in wet pressing. Increasing the concentration of the polymers resulted in increased solids.
Busker L. H., Cronin D. C., The relative importance of wet press variables in water removal, Pulp and Pap Can, 85:87-101 (1984), which is incorporated by reference in its entirety, suggested that additives are not apt to be the most productive areas of research and development for large gains in water removal.
In Wegner T H, The effect of polymeric additive on papermaking, Tappi J 7:107-111 (1987), hereinafter “Wegner,” which is incorporated by reference in its entirety, the effects of cationic polyacrylamide on water removal in the forming, pressing, and drying sections were discussed. Wegner observed that while an increase in drainage was evident, the sheet behavior during wet-press dewatering was unaffected. It was noted, however, that wet pressing with a cationic polyacrylamide could compensate for higher moisture levels entering the press while maintaining the solids content exiting the press.
In Springer A, Nabors L A, Bhatia O, The influence of fiber, sheet structural properties, and chemical additives on wet pressing, Tappi J, 4(2):221-228 (1991), hereinafter “Springer,” which is incorporated by reference in its entirety, chemical additives such as cationic polyacrylamides were shown to have an indirect influence on wet pressing. These chemical additives increased solids exiting the forming section. However, the gains associated with the increased solids were lost during the pressing section. Springer indicated that for sheets entering the press at equal moistures, the additives had no effect on the outgoing solids content. Springer postulated that if an additive was to have any effect on press enhancement, the additive must be able to penetrate the fiber structure and influence its water holding capacity.
What is needed is a process of forming cellulosic products and cellulosic products, capable of dewatering while not suffering from the above-drawbacks.